Time and frequency synchronization is essential for reliable digital communications between the transmitting (Tx) and receiving (Rx) radios. As is known in the art, both transmitter and receiver should have the same nominal frequencies when communicating together. The receiver needs to “tune” within a certain tolerance to receive information exactly on the same frequency as the transmitter is transmitter in order to begin the demodulation process. This is commonly known as frequency synchronization and is required because the reference oscillators in the both radios (Tx and Rx) have different errors from the nominal frequency.
Similarly, time synchronization of incoming digital information is also required since the receiver does not know the boundaries between incoming data symbols. Thus, symbol time synchronization refers to the boundary between successive symbols or digital data bits in order to successfully detect the symbols. This invention addresses the problem of acquiring synchronization (both time and frequency) by using a single 5 millisecond (mS) long synchronization word. The solution for fast acquisition enables the operation of a transmit interrupt feature that is one of the distinguishing features of the new Digital Interchange of Information & Signaling (DIIS) standard that is intended to enable the transition from the analog technology in today's low tier Private Mobile Radio (PMR) systems. This type of system enables a higher speed (12 Kbps) digital communication supporting both speech and data. This is an evolution from an earlier European standard, Binary Interchange of Information and Signaling (BIIS) also known as ETS300.230.PMR protocol (DIIS).
The operation of a sync acquisition system depends on a known sequence of symbols that is periodically embedded in the transmit symbol bit stream. This sequence of symbols, already known to the receiver, is called the synchronization word. Any subsequent call related information is generally sent immediately after the sync word. In this way, any receiver when establishing initial communication starts looking for the sync word and call information to decide whether to participate in the communication or “call”.
The functional diagram of a typical receiver may be similar to the one shown in prior art FIG. 1. A common issue associated with this type of receiver is acquisition time. Acquisition time is the time it takes to sync transmitted data with received data i.e. the time during which the receiver cannot receive data since it is not yet in sync with the transmitted data. Digital in-phase (I) and quadrature (Q) baseband (zero center frequency or low IF or very low IF) input signals 102 are input to a coarse automatic frequency control (AFC) 104 for bringing the range of the radio frequency (RF) input signal within the range of a sharp digital channel select (CS) filter 106.
Although depicted here having a 3 dB bandwidth at 3 KHz for the DIIS modulation, such CS filter is chosen to select the desired signal while rejecting any off-channel power. Without the coarse AFC 104 however, the digital signal might be shifted out of the CS passband in view of the frequency. Typically for DIIS modulation it is required to bring the digital I-Q input signal 102 within 600 Hz of the center frequency of the CS filter 106 or too much signal is lost.
The filtered signal is then passed to frame sync detector 108 which is a device looking for a sequence of digital symbols that is known to the receiver apriori. Thus anytime the receiver detects energy within the IF filter passband, it begins the process of detecting a known sequence of bits for frame symbolization. By using the fine symbol time estimator 110, the receiver determines the boundary between symbols and also achieves frame synchronization (i.e. recognizes the known pattern of incoming bits of information).
Based on the time symbol estimation the receiver 100 will next do a fine frequency estimation to further reduce the frequency error between the transmitter and receiver frequencies. In order to properly decode data it is necessary to make this frequency error smaller than the tolerance of the symbol detection scheme. The tolerance could be as small as 10 Hz in case of coherent detection of DIIS signal or 100 Hz for non-coherent detection of DIIS signal. Since time synchronization has already been achieved, the fine frequency estimation works on known symbols using a fine frequency estimator 112. Since the coarse AFC 104 can only tune the incoming I-Q baseband signal to within 600 Hz, the fine frequency estimator 112 works to fine tune the frequency of incoming data to approximately with 10 Hz in order to property detect the incoming data symbol. This correction is applied to mixer 114 where it is mixed with the signal from the IF filter 106. The output of the mixer 114 is then applied to the symbol detector 116 where it is then properly detected.
The prior art receiver synchronization system as seen in FIG. 1 has several weaknesses. The CS filter with a 3 dB bandwidth at 3 KHz is typically required for meeting an adjacent channel interference protection requirement. With this 3 dB bandwidth, a maximum offset of 600 Hz is acceptable at the input of the IF filter. According to related standards specifications, a mobile transmitter frequency is allowed to be up to 1.5 KHz away from its nominal value for a channel separation of 12.5 KHz. If the baseband I-Q signal is directly fed to the CS filter, in the worst case, with a difference of 3 KHz between Tx and Rx, a significant part of the desired signal gets attenuated by the CS filter. This accounts for the coarse AFC 104 placed before the CS filter 106. The coarse AFC 104 is supposed to bring the filter offset down form 3 KHZ to 600 Hz. The coarse AFC 104 however has to operate on unknown data symbols before the sync word, for the sync word to pass through the IF filter. This ultimately leads to a greater than acceptable delay and a period in which no synchronization occurs where the receiver is unable to receive information.
As seen in FIG. 2, the prior art system shows the frequency of incoming data information being within 3 KHz without any correction at time T0 120. The coarse AFC 104 narrowing the frequency offset within 600 Hz at time 122 and the fine AFC 112 bringing the offset within 100 Hz at time T1 124 for non-coherent symbol detection. This results in a large receiver delay in being able to be on the correct frequency to detect incoming data.
Therefore the needs exists to provide a digital receiver synchronization system that can easily and accurately provide both time and frequency synchronization to an incoming data stream with minimal delay to prevent any loss of incoming digital information.